This invention relates to a PWM inverter apparatus in which an AC voltage having any desired frequency is generated from a DC voltage by PWM switching for controlling the rotation speed of a motor.
An induction motor has the merits of being stout and inexpensive and is presently widely employed, but it has the defect that its rotation speed is principally determined by the frequency of the supplied voltage and is difficult to vary. On the other hand, a variable speed is strongly demanded nowadays for the induction motor (referred to hereinafter as a motor) from the viewpoints of utilities and energy saving, and, as a means for solving this problem, a PWM inverter apparatus is employed in which a commercial AC voltage is converted into a DC voltage, and, by suitable switching, an AC voltage having any desired frequency and voltage is generated again. However, it is known that, when such a PWM inverter apparatus is used, a PWM voltage in pulse form is generated as an output voltage, and a harmonic current is supplied to the motor, with the result that a force having frequencies two times as high as various harmonic components is generated in the primary windings of the motor, and the laminate forming the stator core of the motor is vibrated to generate magnetic noise. At the same time, bipolar transistors are used presently as switching elements for realizing the DC/AC conversion. In view of the properties of such elements, the upper limit of the carrier frequency of the PWM voltage is about 4 kHz, and a carrier frequency of about 2 kHz is commonly used. Accordingly, the motor generates magnetic noise at 16 Hz-20 kHz, which is in the audible range and which it provides a source of discomfort.
Further, in the PWM signal generating part which generates the PWM voltage, a fundamental wave and a triangular wave providing the carrier frequency are compared to generate the PWM signal for each phase. The ratio (=a/b) between the amplitude a of the fundamental wave and the amplitude b of the triangular wave is called the modulation rate m which represents an output voltage relative to an input voltage. There is always the relation a&lt;b, and, theoretically, the output voltage generated is the same as the input voltage when a=b. In order to efficiently drive the motor, it is necessary to maintain constant the ratio of the output voltage/frequency. Thus, the frequency of the fundamental wave is changed while, at the same time, the amplitude a is changed, and the carrier frequency of the triangular wave is selected to be about 2 kHz so as to drive the motor at about the maximum modulation rate m=95%. However, as described already, noise having a frequency two times as high as the carrier frequency is generated, and, especially, when the motor is driven at a low speed, that is, when the noise of the motor itself is low, magnetic noise is heard to an degree. Therefore, studies have been made to increase the carrier frequency thereby generating magnetic noise which exceeds the audible range of a man. However, when the carrier frequency is increased, the minimum output pulse width at the maximum modulation rate provides a frequency about 20 times as high as the carrier frequency. Accordingly, stable generation of the PWM signal at the high carrier frequency and at the maximum modulation rate of 95% is difficult.
Therefore, it is a first feature of the present invention that a PWM inverter apparatus is provided in which the carrier frequency of the PWM voltage is selected to be 8 kHz-24 kHz. Further, it is a second feature that, in such a PWM inverter apparatus, a PWM voltage whose carrier frequency is changed by a set frequency is generated so as to attain noise-free rotation speed control of the motor.